Process for producing pure hydrogen with low steam export

ABSTRACT

A process is proposed for producing pure hydrogen by steam reforming of a feed gas comprising hydrocarbons, preferably natural gas or naphtha, with a simultaneously low and preferably adjustable export steam flow rate. The process includes the steam reforming of the feed gas, for which the heat of reaction required is provided by combustion of one or more fuel gases with combustion air in a multitude of burners arranged within the reformer furnace. According to the invention, the combustion air, before being introduced into the burners, is heated by means of at least one heat exchanger in indirect heat exchange with the hot flue gas to temperatures of at least 530° C.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to EP 20020426.1, filed Sep. 23, 2020,the entire contents of which are incorporated herein by reference.

BACKGROUND Field of the Invention

The invention relates to a process for producing pure hydrogen by steamreforming a feed gas comprising hydrocarbons, preferably natural gas ornaphtha, with a simultaneously low and preferably adjustable flow rateof export steam which is obtained as secondary product in the steamreforming of hydrocarbons.

Prior Art

Hydrocarbons may be catalytically reacted with steam to afford synthesisgas, i.e. mixtures of hydrogen (H₂) and carbon monoxide (CO). As isexplained in Ullmann's Encyclopedia of Industrial Chemistry, SixthEdition, 1998 Electronic Release and 6th edition 2003, keyword “GasProduction”, steam reforming is the most commonly employed method ofproducing synthesis gas which may then be converted to further importantcommodity chemicals such as methanol or ammonia. While differenthydrocarbons, such as for example naphtha, liquid gas or refinery gasesmay be converted, it is steam reforming of methane-containing naturalgas (steam methane reforming, SMR) that dominates. This is highlyendothermic. It is therefore performed in a reformer furnace in whichnumerous catalyst-containing reformer tubes in which the steam reformingreaction takes place are arranged in parallel. The outer walls of thereformer furnace and its ceiling and floor are faced or lined with aplurality of layers of refractory material which withstands temperaturesof up to 1200° C. The reformer tubes are usually fired with burnerswhich are mounted on the top or bottom or on the side walls of thereformer furnace and directly fire the interspace between the reformertubes. The fuel employed is a portion of the feed gas and/or by-productstreams containing flammable components obtained during product workup.Heat transfer to the reformer tubes is affected by thermal radiation andconvective heat transfer from the hot flue gases obtained duringcombustion in the burners.

After pre-heating by heat exchangers or fired heaters to about 500° C.,the hydrocarbon-steam mixture enters the reformer tubes after finalheating to about 500° C. to 800° C. and is converted therein to carbonmonoxide and hydrogen over the reforming catalyst. Common catalysts arenickel-based reformer catalysts which are introduced into the reformertubes as a bed of shaped bodies for example. While higher hydrocarbonsare converted fully to carbon monoxide and hydrogen, partial conversionis typical in the case of methane. The composition of the product gas isdetermined by the reaction equilibrium; the product gas thus comprisesnot only carbon monoxide and hydrogen but also carbon dioxide,unconverted methane and water vapour.

For energy optimization and/or in the case of feedstocks comprisinghigher hydrocarbons, the above-described reforming process, alsoreferred to synonymously hereinafter as main reforming, may have what iscalled a prereformer for precracking of the feedstock arranged upstreamthereof. Prereforming is usually to be understood as meaning the use ofa low temperature reforming step arranged upstream of a conventionalmain reformer operated with natural gas, for example a steam reformer.By comparison with the steam reforming reaction, the reactionequilibrium is established at much lower temperatures in theprereforming. The main feature of the prereforming is the conversion ofthe higher hydrocarbons in the feed mixture to methane and partly tosynthesis gas constituents. A reactor type often used for theprereforming is a simple adiabatically operated shaft reactor filledwith special prereforming catalyst which thus differs markedly also interms of its construction from the allothermically operated mainreformer comprising a multiplicity of reformer tubes.

Since virtually all higher hydrocarbons present in the natural gas usedas feed are converted to methane and synthesis gas constituents in theprereformer, the risk of forming coke deposits in the main reformer ismarkedly reduced. This makes it possible to reduce the steam/carbonratio (S/C) and increase the thermal loading of the reformer tubes, thusresulting in an altogether reduced energy consumption and a reduction insize of the employed apparatuses.

The prereforming stage usually also has a desulfurizing stage arrangedupstream of it to remove sulfur proportions in the feedstock that act asa catalyst poison for the catalysts present in the downstream reformers.The desulfurization may be carried out on a purely adsorptive basis, forexample over adsorbents based on zinc oxide. For some applicationspreference is given to hydrogenating desulfurization(hydrodesulfurization), wherein the sulfur bound in organic andinorganic sulfur components is in the presence of suitable catalystsliberated in the form of hydrogen sulfide using hydrogen andsubsequently bound to adsorbents of the above-described type. Therecited desulfurization methods are therefore often employed incombination.

Before introduction into the reformer tubes filled with reformercatalyst which are arranged in the reformer furnace of the main reformerand heated with burners, the optionally desulfurized and prereformedfeed gas is brought to the steam reforming entry temperature. Thisemploys a feed gas superheater which is configured as a multistage heatexchanger, wherein the heat exchanger stages are located in the wasteheat section for the burner flue gas of the reformer furnace, forexample in the form of so-called coils.

The hot synthesis gas product gas is partially cooled in indirect heatexchange against cold media in one or more heat exchangers after leavingthe reformer furnace. It is possible here to obtain, inter glia, streamsof steam of different quality from aqueous condensates or pure water,for example boiler feed water.

The partly cooled crude synthesis gas then undergoes furtherconditioning steps dependent on the type of the desired product or ofthe downstream process. If the emphasis is on the production of purehydrogen, these typically include a CO conversion plant for reaction ofcarbon monoxide in the crude synthesis gas with steam to give furtherhydrogen and carbon dioxide (also referred to as CO shift or water-gasshift (WGS) plant), a carbon dioxide removal apparatus, for example bygas scrubbing with cryogenic methanol by the Rectisol process, andfinally an apparatus for pressure swing adsorption (PSA) that gives purehydrogen as the end product.

Steam reforming plants thus efficiently produce not only the mainsynthesis gas product or its constituents, for example pure hydrogen,but also a product steam stream consisting under some circumstances ofmultiple steam substreams as a by-product, which is released wholly orpartly as export steam to external consumers. The consumers of the mainproduct here are often not identical to the acceptors of the exportsteam, or can accept only small amounts thereof. Discrepancies can thusoccur between synthesis gas production and the acceptance of the exportsteam since the production rates of the two products, synthesis gas andproduct steam, are coupled to one another, and the production rate ofthe export steam in the conventional steam reforming process can beadjusted only to a very minor degree. Secondly, especially in the caseof part-load operation of the steam reformer, it is under somecircumstances not possible to deliver the volume of export steam agreedwith external consumers. Moreover, it may be desirable to temporarilylower the volume of export steam to be released if, for example,shutdowns in operation or part-load operation occur in acceptorfacilities.

In the case of primary or exclusive production of pure hydrogen by steamreforming, for comparative purposes, the ratio of export steam producedto hydrogen produced is often reported in kg of steam per standard cubicmetre of hydrogen.

The prior art therefore already describes processes for steam reformingof methane in which attempts were made to achieve decoupling of exportsteam production from synthesis gas production. For instance, Europeanpatent application EP 2103568 A2 describes a process for steam reformingof methane, in which virtually no export steam is released to externalconsumers. This is achieved in that, firstly, virtually all the steamproduced is consumed in the reforming process itself, Secondly, one wayin which steam production is minimized is in that the fuel is burnt inthe reformer furnace by means of oxygen-enriched air, which reduces themass flow of hot combustion offgases available for indirect heatexchange with water/steam streams. It is also proposed that the amountof heat transferred in indirect heat exchange against hot synthesis gasproduct gas or hot combustion offgases be used for the superheating of asteam stream that is subsequently utilized for energy generation in asteam turbine. The energy recovered here can then be used in turn foroxygen production or oxygen enrichment of the combustion air. Adisadvantage here is the complex plant concept.

A similar approach for minimization of steam export is followed inpatent publication EP 2103569 A2. As well as the largely completeconsumption of the steam produced within the reformer, the synthesis gasproduct gas, after cooling, is sent to a pressure swing adsorption inwhich carbon dioxide is removed. The resultant hydrogen-enriched productgas from the pressure swing adsorption that thus has an elevatedcalorific value is partly returned to the reformer furnace and used asfuel therein. This is disadvantageous since the high-value hydrogenproduct produced in a complex manner is merely utilized thermally bycombustion.

European patent specification EP 2512981 B1 describes a process forsteam reforming of a hydrocarbonaceous feed. In this case, a portion ofthe flue gas is branched off after leaving the waste heat section of thereformer furnace and before entering the flue gas disposal, and recycledto the reformer furnace. By variation of the proportion of the recycledflue gas, it is possible to vary the amount of export steam released toexternal consumers within a wide range. In this way, it is possible tokeep the amount of export steam constant even in part-load operation ofthe reformer. Alternatively, in the case of full-load operation of thereformer, it is possible to lower or further increase the amount ofexport steam by corresponding alteration of the mass flow rate of fluegas recycled.

In spite of the described approaches from the prior art, there is stilla need for a process for steam reforming of hydrocarbons directed to theproduction of pure hydrogen that simultaneously enables production of alow and preferably adjustable flow rate of export steam. It is notalways desirable or technically possible, for example, to recycle fluegases.

SUMMARY

It is therefore an object of the present invention to specify such aprocess. This object is achieved, in a first aspect, by a process havingthe features of claim 1. Further embodiments of the invention areevident from the pending process claims.

Prereforming conditions, steam reforming conditions and CO conversionconditions are known to those skilled in the art from the prior art, forexample the documents discussed at the outset. These are thephysicochemical conditions under which a measurable conversion,preferably one of industrial relevance, of higher hydrocarbons to lowerhydrocarbons is achieved, in particular methane (prereforming) orhydrocarbons to synthesis gas products (steam reforming or mainreforming), or of carbon monoxide with steam to give carbon dioxide andhydrogen (CO conversion, CO shift).

In the case of steam reforming conditions, important parameters for thispurpose include the establishment of a suitable steam reforming entrytemperature of typically about 1000° C. and addition of steam to thefeed gas containing hydrocarbons and thus adjustment of a steam/carbonratio (S/C ratio). Typical values for the S/C ratio are between 1.5 and3.5 mol/mol. Necessary adjustments of these conditions to the respectiveoperational requirements will be made by those skilled in the art on thebasis of routine experiments. Any specific reaction conditions disclosedmay serve here as a guide, but they should not be regarded as limitingin relation to the scope of the invention.

The statement that the reformer tubes do not comprise any heat recoveryapparatus, especially any internal heat exchanger, should be understoodto mean that the feed gas or product gas flows through the reformertubes in straight pass and then is discharged from them without anydeflection of the gas flow and any introduction into a heat exchangerdisposed within the reformer tube or directly on the reformer tube, i.e.with establishment of physical contact, with the aid of which heat istransferred to the gas flow entering the reformer tube by indirect heatexchange, as proposed, for example, in patent specification EP 2776365B1.

The indication that a material stream is directly supplied to a specificprocess stage or a specific plant part is to be understood as meaningthat the material stream is introduced to this process stage or thisplant part without previously having been passed through other processstages or plant parts with the exception of purely transportationaloperations and the means required therefor, for example pipelines,valves, pumps, compressors, reservoirs.

All pressures are reported in absolute pressure units, barn for short,or in gauge pressure units, barg for short, unless otherwise stated inthe particular individual context.

For the purposes of the present invention, a fluid connection betweentwo regions of the apparatus of the invention is any type of connectionwhich makes it possible for a fluid, for example a gas stream, to beable to flow from the one region to the other of the two regions,regardless of any regions or components located in between. Inparticular, a direct fluid connection is to be understood as meaning anytype of connection which makes it possible for a fluid, for example agas stream, to flow directly from one to the other of the two regions,with no further regions or components being interposed, with theexception of purely transportational operations and the means requiredfor this purpose, for example pipelines, valves, pumps, compressors,reservoirs. One example would be a pipeline leading directly from one tothe other of the two regions.

For the purposes of the present invention, a means is something whichmakes it possible to achieve, or is helpful in achieving, an objective.In particular, means for carrying out a particular process step are allphysical objects which a person skilled in the art would take intoconsideration in order to be able to carry out this process step. Forexample, a person skilled in the art will consider means of introducingor discharging a material stream to include any transporting andconveying apparatuses, i.e. for example pipelines, pumps, compressors,valves, which seem necessary or sensible to said skilled person forperformance of this process step on the basis of such a person'sknowledge of the art.

For the purposes of this description steam is to be understood as beingsynonymous with water vapour unless the opposite is indicated in anindividual case. By contrast, the term “water” refers to water in theliquid state of matter unless otherwise stated in an individual case.

Heat exchange relationship is to be understood as meaning thepossibility of heat exchange or heat transfer between two regions of theapparatus according to the invention for example, wherein any mechanismsof heat exchange or heat transfer such as heat conduction, heatradiation or convective heat transport may come into effect. An indirectheat exchange relationship is especially to be understood as meaning thetype of heat exchange or heat transfer which is carried out through awall (so-called heat transit) which comprises the stages of heattransfer from fluid to the surface of the wall, heat conduction throughthe wall and heat transfer from the surface of the wall to fluid 2.

A further purification, conditioning or processing step of the rawsynthesis gas is to be understood as meaning any measure or process stepknown from the prior art for producing a pure synthesis gas, purehydrogen and/or pure carbon monoxide. These include CO conversion forincreasing the hydrogen proportion in the synthesis gas, separation ofcarbon dioxide by means of a suitable scrubbing process such as forexample the Rectisol process or scrubbing with amine-containingscrubbing media, cryogenic gas fractionation for producing pure carbonmonoxide, pressure swing adsorption (PSA) for producing pure hydrogenand physical process steps such as for example cooling, condensing andseparating the condensate.

For heating of the reformer tubes in the reformer furnace, apart fromrecycled combustible by-product streams from the steam reforming plant,it is also possible to burn a proportion of the feed gas as fuel gas inthe burners for generation of heat. This proportion of what is calledthe trim gas corresponds to the calorific value contribution of the feedgas based on the overall calorific value of the fuel gas. It alsocorresponds to the proportion of the flow rate of the feed gas based onthe total volume flow rate of the fuel gas, corrected by the respectivecalorific values.

The invention is based on the finding that, in a steam reformingprocess, a portion of the enthalpy of the hot flue gases can be utilizedfor preheating of the combustion air to higher temperatures than knownfrom the prior art, rather than utilizing this proportion of theenthalpy for production of additional pure steam from boiler feed waterin a waste heat tank, which can then be released to external consumersas export steam. For this purpose, an appropriate heat exchanger tubewinding (called heat exchanger coil) is introduced into the flue gaspathway of the reformer furnace. In this way, the firing performance ofthe burners can be reduced, leaving the rest of the heat budget of thesteam reforming plant unaffected, and still providing, for example,sufficient amounts of enthalpy for preheating of the other feed streamsfor the steam reforming process, for example the feed gas containinghydrocarbons.

In the context of the invention, the presence of a prereforming stage(prereformer) and the interaction thereof with the other parts of theplant or process steps is particularly advantageous and has a favourableeffect in two ways on the intended reduction in the volume of exportsteam: Firstly, the preliminary cracking of the feed gas containinghydrocarbons in an external prereforming stage upstream of the steamreforming plant reduces the load on the steam reforming plant as themain reforming stage; consequently, it is possible to operate theburners therein at lower load/power. Secondly, the preheating of thefeed gas introduced into the prereforming stage is in turn brought aboutvia indirect heat exchange with the hot flue gases from the reformerfurnace, for which a dedicated heat exchanger coil is again provided.

Both aspects bring about a reduction in the enthalpy of the hot fluegases, such that less pure steam has to be produced as export steam inorder to remove this enthalpy to the desired degree and hence reduce it.A second aspect of the process according to the invention ischaracterized in that the prereformer comprises two prereforming stages,where the individual stages of the prereformer are configured as shaftreactors filled with a solid catalyst active for the prereforming. As aresult of the two-stage preliminary cracking or prereforming of the feedgas, there is an even greater degree than in the case of one-stageprereforming of displacement of the steamcracking to the prereformingstage, and hence a reduction in the load on the main reforming stage onthe one hand and removal of enthalpy from the hot flue gases from thereformer furnace on the other hand. This makes it possible, for a fixedratio of export steam produced to hydrogen produced, to set lowersteam-to-carbon ratios in the feed gas mixture for the steam reformingthan in the case of a purely one-stage prereformer. On the other hand,for a given steam-to-carbon ratio, it is possible to further reduce theratio of export steam produced to hydrogen produced. A third aspect ofthe process according to the invention is characterized in that thereformer tubes are arranged in rows in the reformer furnace, and in thatthe burners are disposed between the rows of reformer tubes and/orbetween the outer rows of reformer tubes and the inner walls of thereformer furnace, where the burners are aligned such that thelongitudinal axis of at least some of the burner flames runs parallel tothe longitudinal axis of the reformer tubes. This achieves aparticularly favourable transfer of heat from the burner flames to thereformer tubes, since the distance of the burner flames from thesurrounding reformer tubes and the length of the respective radiationzones is optimized.A fourth aspect of the process according to the invention ischaracterized in that the reformer tubes do not comprise any heatrecovery apparatus, especially any internal heat exchangers, and theprereformed feed gas flows through in straight pass. Especially in thecase of steam reforming processes that use such conventional reformertubes, the product gases and burner flue gases have a high enthalpycontent that is typically utilized in the form of production of largevolumes of export steam. Therefore, the invention enables particularlyeffective reduction in the volume of export steam.A fifth aspect of the process according to the invention ischaracterized in that the hydrogen production plant comprises adesulfurization stage.A sixth aspect of the process according to the invention ischaracterized in that the hydrogen purification plant comprises at leastone apparatus selected from the following group:

-   -   carbon dioxide removal apparatus    -   apparatus for cryogenic gas fractionation    -   apparatus for pressure swing adsorption (PSA)

The apparatuses mentioned and the processes implemented thereby havebeen found to be effective in the workup of crude synthesis gas to givepure gases.

A seventh aspect of the process according to the invention ischaracterized in that the carbon dioxide removal apparatus is configuredas a scrubbing operation wherein at least one scrubbing agent selectedfrom the following group is used: methanol, N-methylpyrrolidone (NMP),secondary amines, preferably diethanolamine, tertiary amines, preferablymethyldiethanolamine, polyethylene glycol dialkyl ethers, preferablypolyethylene glycol dimethyl ether. Especially after a CO conversionstage that further increases the carbon dioxide content in the synthesisgas, effective removal of carbon dioxide is required since this woulddisrupt the further workup of the synthesis gas to give pure hydrogen.

An eighth aspect of the process according to the invention ischaracterized in that the hydrogen purification plant comprises anapparatus for pressure swing adsorption, and in that at least onecombustible offgas stream is discharged from the apparatus for pressureswing adsorption and is added at least partly to the fuel gas. In thisway, it is possible to sensibly utilize the enthalpy content of thisby-product stream.

A ninth aspect of the process according to the invention ischaracterized in that the crude gas temperature is between 800 and 950°C., preferably between 830 and 900° C., most preferably between 840 and890° C. Studies have shown that, within these ranges of values for thecrude gas temperature, particularly effective lowering of the volume ofexport steam is possible by the invention.

A tenth aspect of the process according to the invention ischaracterized in that the overall SIC ratio is between 2.2 and 3.7,preferably between 2.5 and 3.5. Studies have shown that, within theseranges of values for the overall SIC ratio, particularly effectivelowering of the volume of export steam is possible by the invention.

An eleventh aspect of the process according to the invention ischaracterized in that the fuel gas has a trim gas content, where thetrim gas content constitutes the calorific value contribution of thefeed gas based on the overall calorific value of the fuel gas and wherethe trim gas content is between greater than zero and 20%, preferablybetween 1% and 16%. Studies have shown that, within these ranges ofvalues for the trim gas content, particularly effective lowering of thevolume of export steam is possible by the invention.

A twelfth aspect of the process according to the invention ischaracterized in that the ratio of export steam produced to hydrogenproduced is between 0 and 1 kg of steam per standard cubic metre ofhydrogen, preferably between 0 and 0.4 kg of steam per standard cubicmetre of hydrogen, most preferably between 0 and 0.2 kg of steam perstandard cubic metre of hydrogen. The volumes of export steam mentionedcan usually be released to external consumers without difficulty, or noexport steam at all is produced in one example. The other processparameters are therefore chosen so as to result in these volumes ofexport steam.

A thirteenth aspect of the process according to the invention ischaracterized in that, based on the flow direction of the flue gasstream, the at least one heat exchanger for heating the combustion airin the flue gas waste heat section of the reformer furnace is disposedupstream of the position of a further heat exchanger provided for thesuperheating of steam, and preferably also upstream of the position of afurther heat exchanger provided for the preheating of the feed gas.Optimization calculations by means of process simulation have shown thata particularly favourable pinch point for configuration of the thermalintegration of the overall process is obtained in this way. In addition,it is possible to make the heat exchange area of the corresponding heatexchanger coil particularly small.

A fourteenth aspect of the process according to the invention ischaracterized in that the conduit for the heated combustion air betweenthe heat exchanger and the burners has refractory facing or lining,using at least one thermal insulation material selected from thefollowing group: refractory stone, refractory casting compound orramming compound, mineral fibre mats, self-supporting mineral fibremouldings. In addition, a metallic inner tube may be provided forguiding of the heated combustion air, which is surrounded by the thermalinsulation material, in order in this way to protect the thermalinsulation material from mechanical stresses at high flow rates.

A fifteenth aspect of the process according to the invention ischaracterized in that the conduit for the heated combustion air betweenthe heat exchanger and the burners is configured such that the flow rateof the heated combustion air is at least 30 m/s, preferably at least 50m/s. In this way, the insulated conduit for the heated combustion aircan be made particularly inexpensive, since smaller conduit crosssections can be used, and there is likewise a reduction in the technicalcomplexity for the required pipeline support structure owing to thelower weight. At these high flow rates, it is additionally particularlyfavourable to provide a metallic inner tube for guiding of the heatedcombustion air, which is surrounded by the thermal insulation material,in order in this way to protect the thermal insulation material frommechanical stresses at high flow rates.

A sixteenth aspect of the process according to the invention ischaracterized in that the at least one heat exchanger for heating of thecombustion air is equipped with a regulatable bypass. In this way, it ispossible to keep the volume of export steam constant over wide ranges ofthe hydrogen production capacity of the process, typically 40% to 100%of the nominal hydrogen production capacity. This is advantageousespecially in part-load operation of the process and on startup andshutdown thereof.

A seventeenth aspect of the process according to the invention ischaracterized in that, in part-load operation of the hydrogen productionplant, the crude gas temperature is lowered relative to that infull-load operation and the overall S/C ratio in part-load operation ofthe hydrogen production plant is increased compared to that in full-loadoperation such that the absolute amount of export steam in part-loadoperation remains constant compared to that in full-load operation. Thisaspect brings further advantages with regard to the release of aconstant volume of export steam to external consumers in part-loadoperation of the process. It is particularly favourable here to choosethe preheating temperature of the combustion air such that it is closeto the upper limit of the numerical ranges mentioned, for examplebetween 700 and 790° C., preferably between 710 and 760° C., mostpreferably between 750 and 755° C. Particularly great advantages canadditionally be achieved in the case of combination of this aspect withthe sixteenth aspect of the process according to the invention,according to which the at least one heat exchanger for heating of thecombustion air is equipped with a regulatable bypass.

A further aspect of the process according to the invention ischaracterized in that the entire process condensate is recycled into thesteam reforming process without restrictions. The process condensatewhich is obtained by the generally multistage cooling of the crudesynthesis gas is guided here within a separate process steam system. Theexport steam system here is fully separated from the process condensateand process steam system, such that it is possible to produce exportsteam of the highest quality that is suitable for condensation-typesteam turbines. At low ratios of export steam to pure hydrogen, moreenthalpy is available for complete evaporation of the process condensatethat would otherwise have to be discharged from the process as liquidwaste stream and disposed of in the case of incomplete evaporation.

A further aspect of the process according to the invention ischaracterized in that an advantageous combination with CO conversionprocesses that require less steam input than conventional CO conversionprocesses is possible. An example of this that may be given is themoderate-temperature CO conversion (MT shift) compared to thehigh-temperature CO conversion (HT shift). The combination of theprocess according to the invention with a CO conversion process havinglower steam input additionally improves the energy efficiency of theoverall process. A further improvement in the energy efficiency of theoverall process is additionally obtained when the prereformer isconfigured in two stages, since the two-stage preliminary cracking orprereforming of the feed gas enables lower steam-to-carbon ratios in thefeed gas mixture than in the case of a merely one-stage prereformer. Theratio of export steam produced to hydrogen produced can be reduced tozero with this configuration.

A further aspect of the process according to the invention ischaracterized in that, in addition, a technology for reducing thecontent of nitrogen oxides (NO_(x)) in the burner flue gas is used,since higher air preheating temperatures increase the nitrogen oxideformation. It is possible to use standard apparatuses and processes forreducing NO_(x) levels, for example ultra-low-NO_(x) burners, andcatalytic (SCR) or noncatalytic (SNCR) reduction of NO_(x) by injectionof corresponding reducing agents such as ammonia or urea.

BRIEF DESCRIPTION OF THE DRAWINGS

Developments, advantages and possible applications of the invention arealso apparent from the following description of working and numericalexamples and the drawings. All features described and/or depicted form,either in themselves or in any combination, the invention, regardless ofthe way they are combined in the claims or the back-references therein.

FIG. 1 is an example of the reformer furnace and of the flue gas wasteheat section in a process for producing pure hydrogen by steam reformingaccording to the prior art.

FIG. 2 is an example of the reformer furnace and of the flue gas wasteheat section in a process for producing pure hydrogen by steam reformingaccording to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an example of a process/a plant for producing synthesis gasby steam reforming according to the prior art. A reformer furnace 1 of amain reforming stage contains a multiplicity of catalyst-filled reformertubes 2, wherein for the sake of clarity FIG. 1 shows only four reformertubes. The catalyst used therein is a nickel-based commerciallyavailable steam reforming catalyst. Via conduits 3, 4, 5, 6, 7 and 38,the reformer tubes are charged with preheated hydrocarbonaceous naturalgas as reformer feed gas that has been desulfurized in a desulfurizationstage beforehand and prereformed in a two-stage prereformer (neithershown pictorially in FIG. 1 and FIG. 2). The inlet temperature of thedesulfurized and prereformed reformer feed gas into the reformer tubesis, for example, 500° C.

In addition, steam is added to the reformer feed before it enters thereformer (not shown in FIG. 1), such that there is a definedsteam/carbon ratio (S/C ratio) of between 2.2 and 3.7, preferablybetween 2.5 and 3.5, and of 3 mol/mol in one example. Since steam hasalready been added to the feed gas in the prereformer, for exampleseparately in each prereformer stage, the steam/carbon ratio is usuallyreported as the overall S/C ratio that accordingly relates to the totalamount of steam added to the feed gas.

After conversion of the feed gas in the reformer tubes, the gaseouscrude synthesis gas as reformer product containing hydrogen, CO andunconverted natural gas constituents is withdrawn via conduits 8 andcollection conduit 9 and cooled in a heat exchanger 10 to obtain acooled reformer product and drawn off via conduit 11 and sent to atleast one further purifying, conditioning or processing step (not shownpictorially). In the case of production of pure hydrogen from the crudesynthesis gas, the further purification, conditioning or processingsteps often comprise the conversion of CO to increase the hydrogencontent of the synthesis gas and a usually multistage hydrogenpurification plant, wherein the hydrogen purification plant comprises atleast one of the following apparatuses: carbon dioxide removalapparatus, apparatus for cryogenic gas fractionation, apparatus forpressure swing adsorption (PSA). The process conditions to be employedfor the purpose are known per se to the person skilled in the art. Theoperation of the hydrogen purification plant or the apparatuses presenttherein, especially the apparatus for pressure swing adsorption, affordsone or more combustible gas streams as by-products that are at leastpartly recycled to a multitude of burners 14 in the reformer furnace,where they are incinerated to generate heat. For heating of the reformertubes in the reformer furnace, a portion of the feed gas is usuallyadditionally also incinerated as fuel gas in the burners 14 to generateheat (not shown pictorially). This proportion of what is called the trimgas corresponds to the calorific value contribution of the feed gasbased on the overall calorific value of the fuel gas. It alsocorresponds to the proportion of the flow rate of the feed gas based onthe total volume flow rate of the fuel gas, corrected by the respectivecalorific values.

The crude synthesis gas discharged from the reformer furnace is cooleddown by way of example in indirect heat exchange against a water streamsupplied via conduit 12 from which, by evaporation in a heat exchanger10, a steam stream is obtained, which is discharged via conduit 13. Thewater stream may comprise fresh water or boiler feed water or aqueousprocess condensate which is obtained in the further cooling of the crudesynthesis gas.

The reformer tubes are fired using the multitude of burners 14 that aremounted at the top end of the reformer furnace and fire the interspacebetween the reformer tubes. Preferably, the reformer tubes are arrangedin rows in the reformer furnace, and the burners are disposed betweenthe rows of reformer tubes and/or between the outer rows of reformertubes and the inner walls of the reformer furnace, where the burners arealigned such that the longitudinal axis of at least some of the burnerflames runs parallel to the longitudinal axis of the reformer tubes.

For the sake of clarity the figure shows only five burners. In thepresent example the burners 14 are operated with a mixture of recycledPSA offgas and natural gas feed gas as combustion gas, which is suppliedto the burners via conduits 15, 16 and distributing conduits 17. Thecombustion air is supplied via conduits 18, 19, 20 and 21, preheatedusing heat exchangers 30, 31 and admixed with the fuel in conduit 16. Afan 22 is used for conveying the combustion air.

In reformer furnace 1, heat is transferred to the reformer tubes bythermal radiation and convective heat transfer from the hot flue gases.Once heat transfer is complete the flue gases enter a waste heat section23 of the reformer furnace 1, The flue gases are conveyed through thewaste heat section of the reformer furnace in the extraction draught ofa fan 24 connected to the waste heat section via a conduit 32.

The waste heat section of the reformer furnace further cools the fluegases via a plurality of heat exchangers in the flue gas pathway, withutilization of the enthalpy of the flue gases for producing one or morefurther vapour streams and for multistage preheating of the reformerfeed and the combustion air. According to the prior art, the combustionair is preheated here to temperatures of, for example, less than 530° C.

With regard to the generation of steam, FIG. 1 shows, by way of example,a heat exchanger 25 as the first heat exchanger in flow direction in theflue gas pathway, in which the hot flue gases are cooled in indirectheat exchange against a hydrogen stream introduced via conduit 26,generating a vapour stream which is discharged via conduit 27 and issent to the further utilization or to export to external consumers. Thecooling of the flue gases in heat exchanger 25 may alternatively beeffected against a steam stream, in which case superheated steam is thenwithdrawn via conduit 27.

After passing through the heat exchangers 28 to 31 used for preheatingthe reformer feed and the combustion air, the cooled flue gases exit thewaste heat section of the reformer furnace via conduit 32 and by meansof the fan 24 are sent via conduit 33 to a flue gas disposal 34.

FIG. 2 shows an example of the reformer furnace and of the flue gaswaste heat section in a process for producing pure hydrogen by steamreforming according to the invention. Identical reference numeralscorrespond here to identical structural process and apparatus elements,unless mentioned otherwise in the individual case. Not shown for reasonsof simplification in FIG. 2, but nevertheless present in the example,are the conduit 18 and the fan 22 for the feeding of combustion air andthe apparatus elements downstream of conduit 32 for discharge anddisposal of the flue gas from the waste heat section 23. Likewise notshown, but nevertheless present in the example, are all apparatuselements that were discussed in connection with FIG. 1 but not shownpictorially therein.

The combustion air is now preheated, in the example of FIG. 2, by a heatexchanger 30 which is supplied with combustion air from the environmentvia a conduit 19. The heat exchanger 30, in the example shown, isupstream of the positions of the further heat exchangers in the flue gaswaste heat section. Optimization calculations by means of processsimulation have shown that a particularly favourable pinch point forconfiguration of the thermal integration of the overall process isobtained in this way. In addition, it is possible to make the heatexchange area of the corresponding heat exchanger coils particularlysmall. The combustion air is heated up by means of heat exchanger 30 inindirect heat exchange with the hot flue gas to a temperature between530 and 790° C., preferably between 540 and 760° C., most preferablybetween 550 and 755° C., and then supplied via conduits 20, 16 and 17 tothe burners 14 of the reformer furnace.

The heat exchanger 30, in one example, may also comprise multipleindividual heat exchangers, all of which are disposed in the flue gaswaste heat section of the reformer furnace and serve to heat up thecombustion air in indirect heat exchange with the hot flue gas. In oneexample, at least one of the individual heat exchangers is upstream ofthe positions of the further heat exchangers in the flue gas waste heatsection, and in one example upstream of the positions of all other heatexchangers in the flue gas waste heat section.

Numerical Example

A steam reforming plant was operated with preheating temperatures of thecombustion air of 515° C. (prior art, comparative example, Comp.) and750° C. (invention, Inv.). The obtained ratios of export steam producedto hydrogen produced and other important operating parameters arecompiled in the tables that follow for three sets of different operatingparameters. In one example, the hydrocarbon feed was prereformed in atwo-stage prereformer rather than a one-stage prereformer, with an inlettemperature in the second prereformer stage of 650° C.

In all inventive examples, it was possible to reduce the ratio of exportsteam produced to hydrogen produced to zero. This was possibleespecially in the example with a two-stage prereformer, even though thecrude gas temperature and the overall S/C ratio were kept constant.

LIST OF REFERENCE SYMBOLS

-   -   [1] Reformer furnace    -   [2] Reformer tubes    -   [3]-[9] Conduit    -   [10] Heat exchanger    -   [11]-[13] Conduit    -   [14] Burner    -   [15]-[21] Conduit    -   [22] Fan    -   [23] Waste heat section    -   [24] Fan    -   [25] Heat exchanger    -   [26] Conduit    -   [27] Conduit    -   [28] Heat exchanger    -   [29] Heat exchanger    -   [30] Heat exchanger    -   [31] Heat exchanger    -   [32] Conduit    -   [33] Conduit    -   [34] Flue gas disposal    -   [38] Conduit

Parameters Comp. Inv. Steam export/H2, kg/m3 (STP) (1) 0.4 0.2 Crude gastemperature, ° C. 872 900 Overall S/C ratio 2.9 2.5 Number ofprereforrner stages 1 1 Air preheating temperature, ° C. 515 750Proportion of trim gas, % (2) 15 4 Steam export/H2, kg/m3 (STP) (1) 0.20.0 Crude gas temperature, ° C. 847 867 Overall S/C ratio 3.4 3.3 Numberof prereforrner stages 1 1 Air preheating temperature, ° C. 515 750Proportion of trim gas, % (2) 13 12 Steam export/H2, kg/m3 (STP) (1) 0.30.0 Crude gas temperature, ° C. 885 885 Overall S/C ratio 3.1 3.1 Numberof prereformer stages (3) 2 2 Air preheating temperature, ° C. 515 750Proportion of trim gas, % (2) 25 15 Elucidations (1) steam export(kg/h)/hydrogen production (m3 (STP)/h) (2) & fuel gas calorific valueor burner output in the reformer furnace via trim gas (hydrocarbon feedgas). Residual calorific value or residual burner output via recycledPSA offgas. (3) net temperature of the second prereformer stage: 650° C.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

What is claimed is:
 1. A process for producing a pure hydrogen productgas by steam reforming of a feed gas containing hydrocarbons,comprising: (a) providing the feed gas comprising hydrocarbons, (b)providing a hydrogen production plant comprising (b1) a prereformer,(b2) a steam reforming plant with a reformer furnace, comprising (b21) amultitude of reformer tubes filled with steam reforming catalyst,wherein the reformer tubes have a feed gas inlet for a feed gas mixtureon a first side of the reformer furnace and a product gas outlet for acrude synthesis gas on a second side of the reformer furnace, whereinthe second side is opposite the first side, and further comprising (b22)a multitude of burners, (b3) a CO conversion plant, (b4) a hydrogenpurification plant, (c) adding steam to the feed gas to obtain asteam-feed gas mixture having an overall SIC ratio, (d) introducing thesteam-feed gas mixture into the prereformer, prereforming the steam-feedgas mixture under prereforming conditions to give a prereformed feed gascomprising hydrogen, carbon oxides, methane and higher hydrocarbons,discharging the prereformed feed gas, (e) introducing the prereformedfeed gas into the feed gas inlet of the reformer tubes of the steamreforming plant, heating the reformer tubes by means of the burners,wherein the burners are operated with a fuel gas containing a proportionof the feed gas as trim gas and a proportion of a recycled combustibleoffgas stream, and with combustion air, wherein the operation of theburners produces a hot flue gas, (f) steam reforming the prereformedfeed gas under steam reforming conditions to give a crude synthesis gascomprising hydrogen, carbon oxides and unconverted hydrocarbons andhaving a crude gas temperature, discharging the crude synthesis gas fromthe product gas outlet of the reformer tubes, (g) introducing the crudesynthesis gas into the CO conversion plant, performing the CO conversionunder CO conversion conditions, discharging a converted crude synthesisgas having elevated hydrogen content relative to the crude synthesisgas, (h) introducing the converted crude synthesis gas into the hydrogenpurification plant, discharging a pure hydrogen product gas and at leastone combustible offgas stream, wherein (i) the combustion air prior tointroduction into the burners is heated up by means of at least one heatexchanger in indirect heat exchange with the hot flue gas to atemperature between 530 and 790° C.
 2. The process according to claim 1,wherein the prereformer comprises two prereforming stages, where theindividual stages are configured as shaft reactors filled with a solidcatalyst active for the prereforming.
 3. The process according to claim1, wherein the reformer tubes are arranged in rows in the reformerfurnace, and in that the burners are disposed between the rows ofreformer tubes and/or between the outer rows of reformer tubes and theinner walls of the reformer furnace, where the burners are aligned suchthat the longitudinal axis of at least some of the burner flames runsparallel to the longitudinal axis of the reformer tubes.
 4. The processaccording to claim 1, wherein the reformer tubes do not comprise anyheat recovery apparatus, and the prereformed feed gas flows through instraight pass.
 5. The process according to claim 1, wherein the hydrogenproduction plant comprises a desulfurization stage.
 6. The processaccording to claim 1, wherein the hydrogen purification plant comprisesat least one apparatus selected from the following group: carbon dioxideremoval apparatus apparatus for cryogenic gas fractionation apparatusfor pressure swing adsorption (PSA)
 7. The process according to claim 6,wherein the carbon dioxide removal apparatus is configured as a gasscrubbing operation wherein at least one scrubbing agent selected fromthe following group is used: methanol, N-methylpyrrolidone (NMP),secondary amines, tertiary amines, polyethylene glycol dialkyl ethers.8. The process according to claim 1, wherein the hydrogen purificationplant comprises an apparatus for pressure swing adsorption, and in thatat least one combustible offgas stream is discharged from the apparatusfor pressure swing adsorption and is added at least partly to the fuelgas.
 9. The process according to claim 1, wherein the crude gastemperature is between 800 and 950° C.
 10. The process according toclaim 1, wherein the overall SC ratio is between 2.2 and 3.7.
 11. Theprocess according to claim 1, wherein the fuel gas has a trim gascontent, where the trim gas content constitutes the calorific valuecontribution of the feed gas based on the overall calorific value of thefuel gas and where the trim gas content is between greater than zero and20%.
 12. The process according to claim 1, wherein the ratio of exportsteam produced to hydrogen produced is between 0 and 1 kg of steam perstandard cubic metre of hydrogen.
 13. The process according claim 1,wherein, based on the flow direction of the flue gas stream, the atleast one heat exchanger for heating the combustion air is disposedupstream of the position of a further heat exchanger provided for thesuperheating of steam.
 14. The process according to claim 1, wherein theconduit for the heated combustion air between the heat exchanger and theburners has refractory facing or lining, using at least one thermalinsulation material selected from the following group: refractory stone,refractory casting compound or ramming compound, mineral fibre mats,self-supporting mineral fibre mouldings.
 15. The process according toclaim 1, wherein the conduit for the heated combustion air between theheat exchanger and the burners is configured such that the flow rate ofthe heated combustion air is at least 30 m/s.
 16. The process accordingto claim 1, wherein the at least one heat exchanger for heating of thecombustion air is equipped with a regulatable bypass.
 17. The processaccording to claim 1, wherein, in part-load operation of the hydrogenproduction plant, the crude gas temperature is lowered relative to thatin full-load operation and the overall S/C ratio in part-load operationof the hydrogen production plant is increased compared to that infull-load operation such that the absolute amount of export steam inpart-load operation remains constant compared to that in full-loadoperation.